The present invention concerns the offset printing of webs with variable sizes.
FIG. 1 of the attached drawing represents a simplified diagram illustrating the operation of an offset printing machine comprising:
a cylinder PP carrying a metal plate P on which the image is engraved;
a cylinder PB carrying a rubber plate called a blanket B and destined to transfer the image by means of an ink film on the corresponding area of what will hereafter be called a travelling web F of a cardboard or similar matter;
a counter-cylinder CC destined to press the web F against the cylinder PB;
an inking unit E comprising an ink roller E.sub.1 with adjustable doctor blades, a dipping roller E.sub.2, distributing rollers E.sub.3, and inking rollers E.sub.4 ; and
a moistening unit M (for water or alcohol) comprising a basin M.sub.1, a dipping roller M.sub.2, distributing rollers M.sub.3, and moistening rollers M.sub.4.
If an offset machine is to be used to the maximum of its potential, it is an established fact that the three cylinders PP, PB, and CC are to have an identical perimeter corresponding to the length of the printing size.
When unrolling over the advancing web F, the plate P of the cylinder PP will transfer one or several print motifs contained within the limits of the entire printing size, and corresponding to a future package. So, for instance, in the event of large washing powder boxes having to be printed, one print might correspond to a circumference of the cylinder or to a printing size (the dead area used for fastening the plates and blankets being deducted); on the other hand, with liquid boxes, several print motifs will be necessary to ensure the entire cylinder revolution. However, when printing, this difference will only enter into account if, at a further stage, the printing size is cut into several parts or blanks whereupon every single motif will be destined to a single package. Consequently, every new run, with a size length different from the preceding one, requires the exchange of the three cylinders PP, PB, CC. Considering that a printing machine might consist of up to ten cylinder assemblies PP, PB, CC, i.e. one assembly for each color, this means that up to thirty cylinders will have to be exchanged for every new run. According to prior art, it is possible to fit the three cylinders for each run and the assembly into a cassette consisting of rigid frames, high-quality gears, crossbars, bearings, inter-axial setting devices and the three cylinders PP, PB, CC (only the essential items being mentioned). With every changing size, the cassette will thus have to be exchanged on every printing unit.
The shortcomings of this procedure can be itemized as follows:
high expenses involved with every exchange;
considerable time required for preparing a new run, involving gear cutting, hardening, and polishing;
long down times caused by every exchange;
high investment frozen for storage.